This invention concerns improvements in or relating to optical apparatus and relates more particularly to low light level or night vision apparatus and especially, but not exclusively, night vision goggles.
Night vision goggles, in which incident light at a low level is intensified to produce a visible image of a viewed scene or object and the image is seen by both eyes of a binocular observer, can be used for example by the pilot of a helicopter or other aircraft. In such mode of use the problem is encountered that the objective lens of the goggles is focussed at infinity for normal flying, and if the pilot looks down at his instruments, which are generally at a distance of about 0.5 to 1 meter away, they will appear blurred and out of focus. The effect is usually very marked because of the large diameter of the goggles relative to the eye. Manual refocussing can be employed but this is inconvenient and slow.